1. Field of the invention:
This invention relates to a semiconductor laser device which emits laser light from an end facet thereof and it also relates to a method for the production of such a semiconductor laser device.
2. Description of the prior art:
In recent years, semiconductor laser devices have widely come into practical use as light sources for optical disc driving units. When semiconductor laser devices are used as the light source for write-once optical disc driving units or rewritable optical disc driving units, they are required to have high reliability even at a high output power level of from 40 to 50 mW. When used as the light source for optical pumping of solid-state laser devices such as a YAG laser, an output power of 100 mW or more is required.
However, it has been reported that the reliability of the semiconductor laser devices in practical use today, which can attain laser oscillation at a relatively high output power level, is inversely proportional to the optical output power raised to the fourth power, when the devices of the same construction are compared. In other words, it is extremely difficult to raise the optical output power, while maintaining high reliability.
The principal cause for deterioration of semiconductor laser devices in high output power operation is facet deterioration. This is because heat is generated locally at the light-emitting facet due to the high optical density at this facet. The mechanism of this heat generation will be explained by reference to FIGS. 5a-5b and 6a-6b.
FIGS. 5a and 5b are schematic diagrams showing the energy band structures near the surface originating from the surface state which occurs when either the n-type or p-type GaAs (110)-surface is slightly oxidized. In the case of either n-type or p-type GaAs, numerous carriers accumulate near the surface to form a so-called "accumulation layer" which is indicated by reference numeral 1 in these figures.
In general, it is well known that the surface state will bend the energy bands near the surface. In addition to the accumulation layer 1 shown in FIGS. 5a and 5b, minority carriers may gather near the surface and majority carriers be distanced from the surface as shown in FIGS. 6a and 6b, resulting in the formation of an inversion layer 2 which is a local inversion of the conductivity type. Whether an accumulation layer 1, or an inversion layer 2 forms, is dependent on the height relationship between the surface state E.sub.s and the Fermi level E.sub.f of the semiconductor. In either case of n-type or p-type GaAs, an accumulation layer 1 will form.
The electrons and positive holes trapped in the surface state E.sub.s are released after a short relaxation time, and this energy is released as heat. Electrons and positive holes are then again trapped in the surface state which has become a vacant state, and the above process is repeated, so that heat continues to be released.
While the above process is being repeated, the heat released from the surface state concentrates at the end facets of the semiconductor, and this heat narrows the forbidden band gap in the energy bands. Furthermore, the absorption of light increases the minority carriers, and heat generation further increases by way of the surface state. This process raises the temperature of the semiconductor surface, which may reach the melting point of the semiconductor, resulting in facet breakdown.
In the case of GaAs, an accumulation layer forms, while in the case of other materials such as AlGaAs, an inversion layer may form. In the latter case, majority carriers are trapped in the surface state, and facet breakdown occurs by the same process as that by an accumulation layer. In the case of semiconductor laser devices used under a high injection condition, heat generation originating with the surface state becomes a more serious problem.
As a method for preventing the effect of surface recombination, there has been proposed a method in which a region having a graded band gap is formed near the surface. For example, a solar cell having a graded-band-gap layer formed by liquid phase epitaxy is disclosed by M. Konagai and K. Takahashi, J. Appl. Phys. Vol. 46 No. 8, pp. 3542-3546 August (1975).
FIG. 7 shows a schematic energy band diagram of a graded-band-gap heterojunction solar cell described in the above-identified prior art reference. As can be seen from this figure, among carriers generated by the absorption of light, electrons are drawn strongly into the inside of semiconductor crystals due to the drift caused by the grading of the energy gap, as well as the migration usually caused by the diffusion. This results in a reduced probability IO that these electrons will be trapped in the surface state, and therefore, surface recombination can be effectively suppressed.
In the case of solar cells, because the crystal surface functions as a light-receiving facet, a graded-band-gap layer can be formed by liquid phase epitaxy during the crystal growth, thereby attaining the effective suppression of surface recombination.
However, in the case of semiconductor laser devices, a light-emitting facet, which may be adversely affected by surface recombination, is usually formed by a cleavage of semiconductor crystals. Therefore, it is extremely difficult to form a graded-band-gap layer in advance on the cleavage plane by liquid phase epitaxy. For this reason, conventional semiconductor laser devices have a serious problem of causing facet breakdown due to surface recombination at the light-emitting facet.